The present invention relates generally to video memory management in MPEG ("Moving Picture Experts Groups") decode and display system, and more particularly to reducing the size of video memory needed in a MPEG decode and display system for decoding and displaying video images.
In the late 1980s, a need arose to place motion video and its associated audio onto first generation CD-ROMs at 1.4 Mbits/s. For this purpose, in the late 1980s and early 1990s, the ISO ("International Organization for Standardization") MPEG committee developed digital compression standards for the video and two-channel stereo audio. The standard is known colloquially as MPEG-1 and officially as ISO 11172.
Following MPEG-1, the need arose to compress entertainment TV for such transmission media as satellite, cassette tape, over-the-air, and CATV. Thus, to have available digital compression methods for full-resolution Standard Definition TV (SDTV) pictures or High Definition TV (HDTV) pictures, ISO developed a second standard known colloquially as MPEG-2 and officially ISO 13818. The bitrate chosen for optimizing MPEG-2 was 4 Mbits/s and 9 Mbits/s for SDTV and about 20 Mbits/s for HDTV.
Neither the MPEG-1 nor the MPEG-2 standards prescribe which encoding methods to use, the encoding process, or details of encoders. These standards only specify formats for representing data input to the decoder, and a set of rules for interpreting these data. These formats for representing data are referred to as syntax and can be used to construct various kinds of valid data streams referred to as bitstreams. The rules for interpreting the data are called decoding semantics. An ordered set of decoding semantics is referred to as a decoding process.
The MPEG syntax supports different encoding methods that exploit both spatial redundancies and temporal redundancies. Spatial redundancies are exploited by using block-based Discrete Cosine Transform ("DCT") coding of 8.times.8 pixel blocks followed by quantization, zigzag scan, and variable length coding of runs of zero quantized indices and amplitudes of these indices. Quantization matrix allowing perceptually weighted quantization of DCT coefficients can be used to discard perceptually irrelevant information, thus increasing the coding efficiency further. On the other hand, temporal redundancies are exploited by using motion compensated prediction, forward prediction, backward prediction, and bidirectional prediction.
The MPEG provides two types of video data compression method: intraframe coding, and interframe coding.
The intraframe coding is for exploiting the spatial redundancies. Many of the interactive requirements can be satisfied by the intraframe coding alone. However, in some video signals with low bitrates, the image quality that can be achieved by intraframe coding alone is not sufficient.
Therefore, the temporal redundancy is exploited by MPEG algorithms which compute an interframe difference signal called the Prediction Error. In computing the prediction error, the technique of motion compensation is employed to correct the prediction for motion. As in H.261, the Macroblock (MB) approach is adopted for motion compensation in MPEG. In unidirectional motion estimation, called Forward Prediction, a Target MB in the picture to be encoded is matched with a set of displaced macroblocks of the same size in a past picture called the Reference picture. As in H.261, the Macroblock in the Reference picture that best matches the Target Macroblock is used as the Prediction MB. The prediction error is then computed as the difference between the Target Macroblock and the Prediction Macroblock.